A large number of industrial processes cause more or less arsenic containing effluents, which cannot effectively be processed with removal of arsenic during waste purification processes with present technology.
In the production of lead crystal and glass, arsenic trioxide (As.sub.2 O.sub.3) is added, up to an amount of 2 mass-% of the molten glass, as a refining agent. Annually, 400 metric tons of arsenic are used for this purpose in the Federal Republic of Germany. Due to present technical limitations, the arsenic cannot simply be substituted with another, safer purifying agent. While part of the used arsenic escapes from the molten glass, an amount of approximately 0.2 mass-% remains in the combined state in the glass matrix.
However, during refining processes in which the surface is worked on, arsenic is again released. This happens in the so-called acidic polishing process, during which the glass is treated with sulfuric acid--hydrofluoric acid mixtures and is partially dissolved again, as well as during the process of cutting glass. Acidic polishing baths contain approximately 40-100 mg As/l while waste from cutting procedures contains typically approximately 1 mg As/l or when recirculated up to approximately 10 mg As/l.
Furthermore, arsenic containing lyes occur when producing nonferrous metals. As an example water soluble arsenic is found in soda slag of pyrometallurgical processes with a concentration up to 4000 mg As/l. Solutions with relatively high concentrations, around several mg/l of arsenic, also occur during wet waste gas purification, such as the desulfurization of flue gas, and in eluates of combustion residues. Waste material containing arsenic also results from the production of micro chips when etching the gallium arsenide used therefor.
Due to the high toxicity of arsenic there is a great need to diminish the content of arsenic in waste material as much as possible.
Experiments regarding the removal of arsenic from effluents produced by the glass industry when cutting glass are described in the Research Report 10207001/06, dated Aug. 1986, produced by the Fraunhofer-Institute for Silicate Research in Wurzburg, Federal Republic of Germany. This research was conducted within the Plan for Environmental Research by the Federal Minister for Domestic Affairs of the Federal Republic of Germany. Industrial effluents such as these contain up to approximately 3 mg As/l and up to approximately 10 mg Pb/l as well as cooling agents, lubricating agents and tensides. The pH-value of such effluents is between 7 and 8.
Within the scope of these experiments several different procedure possibilities for separating arsenic from waste have been tested systematically. Aside from experiments of reducing arsenic on the surface of metals, such as iron and zinc, other experiments were also conducted to precipitate arsenic as arsenic sulfide, as calcium arsenate, as ferro arsenate, as magnesium arsenate, as aluminum arsenate, and as lead arsenate. In all of those cases only a separation degree of less than 50% could be obtained.
Further experiments examined separation by means of adsorption to silica gel containing titanium oxide, alumina and active carbon. The above Research Report at page 14 states that no relevant decrease of the arsenic content in waste was noticeable when using silica gel and active carbon. Only the use of aluminum oxide resulted in a minimal decrease of the arsenic content. However, this reduction is too small for a practical application.
The only somewhat promising separation method of arsenic among all the systematically conducted experiments turned out to be the precipitation with ferric salts with or without previous oxidation of As(III) to As(V). This procedure resulted in a decrease of arsenic from 4 mg/l to less than 0.1 mg/l in suitable effluents. In waste material produced by acidic polishing in the glass industry, which can contain arsenic up to 100 mg/l in strongly acidic solutions, the content of arsenic could only be reduced by one half through precipitation of sulfide of arsenic, through dissolution of iron sulfide in acidic waste (see page 50 of the Research Report).
In other words, samples with an initial content of arsenic of approximately 15 mg/l subsequently still contained 6 to 7 mg/l of arsenic. Due to the extensive amount of experiments regarding the separation of arsenic (see page 50 of the Research Report) it is thus established that according to current technology even an elaboration of a proposal of a possible technical procedure for separating arsenic requires further laboratory experiments. The Research Report, on page 51, points out that there is no existing proposal for a procedure that will separate arsenic from effluents produced by acidic polishing.
In the publication "Ecologically Harmless Technologies For The Production Of Lead Crystal And Crystal--Removal Of Arsenic From Effluents Produced When Cutting Glass" of the Fraunhofer Institute for Silicate Research, Wurzburg, Federal Republic of Germany, the separation of arsenic from waste is described as a formation of ferric arsenate under coprecipitation in a ferric hydroxide precipitation which occurs according to the following gross reaction equation: EQU Fe.sup.3+ +3H.sub.2 O.fwdarw.Fe(OH).sub.3 .dwnarw.+3H.sup.+ EQU Fe.sup.3+ +AsO.sub.4.sup.3- .fwdarw.FeAsO.sub.4 .dwnarw..
To avoid a decrease of the pH-value due to the added iron sulfate, solution a calcium hydroxide suspension is used for neutralization, whereby the sulfate ions are precipitated as gypsum. This procedure allows, however, only for glass making waste with small amounts of arsenic, for a reduction of the arsenic content up to an amount below 0.1 mg/l, but this is not applicable, for example to effluents from acidic polishing procedures. In these cases only 50% of the arsenic content can be separated.
The pamphlet ATV-VKS M352 (dated Apr. 1987) on the "Treatment and Removal of Scorodite Mud" suggests to transform water soluble arsenic from pyrometallurgic procedures (scorodite=ferric arsenate FeAsO.sub.4) in a concentration of 1000 to 4000 mg/l in a sulfuric acid solution into arsenic(V) with the aid of chlorine, and to precipitate it at temperatures of 60.degree. to 70.degree. C. and pH-values of less than 1 by means of adding ferric salts and subsequent careful raising of the pH-value to 2 to 2.5 with calcium hydroxide. The article does not give any remarks on the efficiency of the precipitation or the remaining concentration of dissolved arsenic. In some processes, 0.3 to 2 mg As/l, 50 to 300 mg Cu/l, 100 to 300 mg Zn/l, 1 to 3 mg Pb/l, 0.1 to 1 mg Fe/l as well as 500 mg Cl/l have been found in the eluates. The amounts of sulfate and calcium correspond approximately to the solubility of plaster. This publication also points out that no alkaline reactions and no reduction procedures should be carried out in order to not increase the solubility of arsenic.
The separation of arsenic from waste according to the above described procedure, the precipitation as ferric arsenate, is further also the subject of German patent application publications 3,632,138 and 3,633,066, with or without previous oxidation of the trivalent arsenic.
The separation of arsenic from waste by means of precipitation of ferric arsenate is thus only applicable to certain, suitable effluents and is not a generally suitable separation procedure for arsenic.
German patent application publication 3,637,643 further describes a procedure for the arsenic decontamination of aqueous solutions, which is based on the oxidation of arsenic(III) to arsenic(V) and the pressure filtration of waste after adding a water soluble polymeric anion exchanger through a membrane. In this process, the arsenic is enriched above the membrane.
According to this method the anion exchangers are polymers with molar masses of 30,000 to 100,000 in connection with membranes of a preclusion limit of 10.sup.4, for example, polyethylenimines. The arsenic concentration of the solution to be decontaminated must not be higher than 2 mg/l if this procedure is applied in one step. Due to the method of membrane filtration used here, this procedure is basically not suitable for an application on a industrial scale.